Optical demultiplexers are frequently used in optical communication systems. One type of optical demultiplexer is an arrayed waveguide grating (AWG), which includes an input to a first free space region, a plurality of channel waveguides, a second free space region and a plurality of outputs. An AWG may be configured to receive a wavelength division multiplexed (WDM) optical signal having a plurality of channels, each at a particular wavelength. The WDM signal propagates through the first free space region into the plurality of channel waveguides. These waveguides have different lengths and thus, each signal undergoes a different phase shift as it exits the waveguides into the second free space region. The light from the second free space region interferes at the output of the AWG such that each output receives only light having a particular wavelength. Because the refractive indices of the channel waveguides may be changed with the application of heat, thin film heaters have been employed to modify the phase of the propagating light through these waveguides to modify the particular wavelengths selected by the AWG. However, the temperature of the heaters is usually configured during implementation within a transmission system based upon a particular spectral spacing of the WDM signal wavelengths and cannot be changed to tune the AWG to select wavelengths having a different spectral spacing.
Mach Zehnder Interferometers (“MZ”) have also been employed as demultiplexers. Generally, an MZ includes a first beam splitter that splits an incoming light signal into two parts and supplies each part onto a respective one of a pair of optical waveguides that may have varying lengths (asymmetric). The split light is then recombined by a second beam splitter and, depending on the relative phase acquired by the light along the two waveguides, the light may undergo constructive or destructive interference. MZs may be configured to separate individual wavelengths supplied to the input of the MZ and/or a plurality of MZs can be cascaded to separate groups of wavelengths from a WDM signal. However, MZs also suffer from the lack of tunability to select different wavelengths or groups of wavelengths from a WDM optical signal
Other optical demultiplexers that may be employed to select particular wavelengths in optical transmission systems include a cascade of Bragg gratings. However, a cascade of Bragg gratings, like an AWG or a cascade of MZs, suffers from the drawback that the demultiplexed output wavelengths are fixed at the time the demultiplexer is fabricated. Accordingly, these conventional demultiplexers only operate with certain wavelengths and have poor response characteristics. In particular, if the wavelengths selected to be demultiplexed from a WDM signal vary from the wavelengths that the demultiplexer is tuned to select, then cross-talk between the channels and an associated power loss occurs thereby compromising the optical transmission system. Accordingly, a tunable optical demultiplexer for use in optical communication systems is desirable to overcome these drawbacks.